Hi, I have a problem with the heating that just started a few days. When I am driving the van, and the heat is on, it will blow hot (from the defrost, lower leg parts, etc.) but after about 20-30 minutes it will blow cold. The first time it happened I thought that the heater core had gone, but next day the exact same thing happened. There is no antifreeze leak at the firewall, or anywhere else for that matter so I don't think it is the heater core. I have done no work on the cooling system, so I don't know where to start. The antifreeze level is good.